This invention relates generally to d.c.-to-d.c. converters which convert one direct-current voltage into another, and particularly to a d.c.-to-d.c. converter for providing a constant output voltage through feedback control.
The d.c.-to-d.c. converter of the kind under consideration comprises a transformer having a primary winding connected across a d.c. power supply via an on/off switch, a rectifying and smoothing circuit connected to the secondary winding of the transformer, an output voltage detector circuit, another detector circuit for detecting the current flowing through the serial connection of the transformer primary and the switch, and a switch control circuit for on-off control of the switch according to the outputs from the output voltage detector circuit and switch current detector circuit.
One of the problems encountered with this type of d.c.-to-d.c. converter was the relationship between the switching frequency of the d.c. voltage and the load connected across the transformer secondary via the rectifying and smoothing circuit. The proportion of power loss at the switch to the power requirement of the load increased when the switching frequency at the time of a drop in power consumption of the load was left as high as, or even higher than, under normal power consumption.
Japanese Unexamined Patent Publication No. 2000-23458 represents a solution to this poor efficiency of the prior art d.c.-to-d.c. converter under light load. The solution is such that the converter is operated at intervals under light load. Switching loss is lessened through reduction of the switching frequency at the sacrifice of stability in output voltage.
The solution has proved not totally satisfactory, however. With the converter put to intermittent operation, the conducting period of the switch inordinately increased at the beginning of each operating period in order to compensate for a drop in output voltage during the preceding nonoperating period. The prolonged closure of the switch invited large current flow through the switch and the transformer. The result, to the discomfort of the user, was the production of magnetostrictive noise by the transformer.
The noted prior art converter relies for its intermittent operation upon on-off control of the voltage feedback by pulses of rectangular shape. These pulses are not exactly rectangular in shape but modified to suppress, to some extent, the magnetostrictive noise, or ringing, of the transformer through restriction of an increase in current flow through the switch during intermittent operation. However, no truly satisfactory suppression of transformer ringing has proved possible in this manner unless, impractically, the feedback control circuit is designed with full consideration to such factors as fluctuations in, and temperature dependencies of, the performance characteristics of its individual components. It is also a disadvantage that an additional pulse generator has had to be provided on the output side of the transformer for intermittent operation of the converter.
The present invention aims, in a d.c.-to-d.c. converter of the kind under consideration, at reduction of the magnetostrictive noise of the transformer or other inductance means to an absolute minimum without the inconveniences experienced heretofore.
Stated in brief, the present invention concerns a d.c.-to-d.c. converter of the general construction comprising a switch connected between a pair of converter input terminals via inductance means such as a transformer, a rectifying and smoothing circuit connected to the inductance means for providing a d.c. output voltage to be applied to a load, an output voltage detector circuit for detecting the output voltage, and a switch current detector circuit for putting out a voltage indicative of the magnitude of a current flowing through the switch.
More specifically, the invention provides, in the d.c.-to-d.c. converter of the type defined above, a periodic wave generator circuit for generating a sawtooth or like periodic wave voltage. In any event the periodic wave voltage should be in the form of a series of alternations of a rise and a fall, with a period that is a plurality of times longer than the period of switching pulses of variable durations applied to the switch. Further the periodic voltage should cross the output voltage of the switch current detector circuit during its rises and falls. A comparator has an input connected to the switch current detector circuit, and another input connected to the periodic wave generator circuit, for providing an output that changes from one state to another depending upon the relative magnitudes of the switch current detector output voltage and the periodic voltage. Also included is a switch control circuit having an output connected to the switch for delivering the switching pulses thereto, an input connected to the output voltage detector circuit for controlling the durations of the switching pulses according to the converter output voltage so as to keep the same constant, and another input connected to the comparator for permitting or preventing the delivery of the switching pulses to the switch depending upon the binary output from the comparator.
It is to be appreciated that the periodic wave generator circuit functions both for the desired intermittent operation of the converter under light load and for current control through the transformer or like inductance means. Thus making the overall converter circuitry materially simpler than heretofore, the periodic wave generator circuit nevertheless attains the seemingly self-contradictory objectives of intermittent converter operation and the suppression of magnetostrictive noise in the course of such operation.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings showing the preferred embodiments of the invention.